The present invention relates to cement compositions for extrusion and, more specifically, to those cement compositions for extrusion which contain pulp fiber. These cement compositions for extrusion provide excellent moldability and workability using binders in quantities that are equivalent to or less than is conventional.
Asbestos fiber reinforced cement boards or plates have so far been widely used as materials for outer walls, roofs, floors of buildings and so forth. The cement boards or plates may be obtained by adding water to a mixture of cement on one hand and asbestos fiber and aggregates, etc. on the other on the order of 20 weight %, then kneading and extruding the entire mixture into a form by means of required dies. The asbestos fiber thus used is characterized by an excellent dispersing quality when kneading, and the kneaded materials obtained have a good shape and water retention when extruded. Further, the heat resistance of the asbestos fiber displays to the fullest extent its performance as reinforcing fiber in the curing by a steam autoclave at 170.degree. C. or a higher temperature to furnish the extruded materials with dimensional stability and required strength. Since the asbestos fiber is an unrivaled reinforcing fiber which satisfies, at the same time, the dispersing quality, shape retention, water retention and thermal resistance required when extruding, they have thus far been used exclusively for extrusion.
In recent years, however, the asbestos fiber has become a social problem because of its carcinogenicity which is believed to be derived from the fibrous shape peculiar thereto. Therefore in an extrusion such as above asbestos is not longer used.
As a substitute for the asbestos fiber, some synthetic fibers were considered which, however, are poor in dispersing quality, shape and water retention. Furthermore, these organic fibers for the most part do not have good heat resistance, some being unable to be cured by the foregoing steam autoclave. The general trend is that the more excellent the reinforcing effect the more expensive such fibers are as compared with the asbestos fiber. With this as background, we are now in such a situation where really satisfying materials cannot be found as substitutes for asbestos fiber.
On the other hand, the pulp fiber, which is one of the organic but natural fibers, is less expensive and better than other organic fibers, though it has an inferior dispersing quality compared to the asbestos fiber. The extruded materials to which a certain quantity of this fiber has been added are advantageous in it that they allow for such processing as nailing and sawing which could not be easily done on the conventional cement moldings containing asbestos fiber. A feature thereof facilitates to mount them as facing materials of buildings. They are now attracting a good deal of public attention as substitute materials for the conventional asbestos fibers.
In the extrusion process of cement mortar compositions, much pressure is in general applied thereonto when molding them. For this reason, such water retention characteristics that the asbestos or pulp fibers in the cement compositions have are not sufficient to hold water uniformly in the moldings. Since in this case the compositions are made to adhere to each other with the water separated before the extruded moldings are set and solidified, it was difficult to maintain the moldings discharged from these dies in form. It has been proposed to use organic type binders in the compositions for extrusion molding in order to improve their moldability. For example the Japanese Patent Publication No. 43-7134 discloses a technology by which binders such as methylcellulose and hydroxypropyl methylcellulose can be used as cement compositions containing an asbestos fiber for extrusion moldings. This method is generally adopted in use of binders. As such binders, in addition to the methylcellulose and hydroxypropylmethylcellulose disclosed in said publication include; hydroxyethylcellulose, carboxymethyl hydroxyethylcellulose, polyethylene oxide, sodium polyacrylate, casein and so forth. Among those, the alkylcellulose and/or hydroxyalkylalkylcellulose, which are excellent in dispersing quality, water retention and tackiness, have been widely utilized in cement and aggregates.
So far several propositions have been offered for compositions containing the pulp fiber for molding to which such binders as above were added. In the Japanese Patent Provisional Publication No. 55-51748, for example, a composition has been disclosed comprising a glass fiber to which a small quantity of pulp (cellulose) fiber and a binder such as methylcellulose have been added. Another Japanese Patent Publication, No. 63-1276, discloses that a molding allowing for riveting and sawing can be obtained by extruding a cement composition to which a pulp (cellulose) fiber is added, one-shot foamed spherical resin and such binders as methylcellulose.
It should be noted, however, that the use, as a binder, of such alkylcellulose and/or hydroxyalkylalkylcellulose similar to those used in conventional cement containing asbestos fiber to be added to the compositions having pulp fiber, is problematical in that it requires more loadings than is conventional to obtain a regular and smooth extrusion molding.
In the extrusion molding of the conventional cement compositions containing asbestos fiber, the cost ratio of required binder or binders to the total cost of the extruded molding is on the order of one fourth to one half. This implies that increased loads of binders will raise the total cost of materials to be molded, which places a high hurdle to the extrusion molding of the cement compositions containing the pulp fiber.
With this as a background it has been strongly desired to have such binders which allow for an excellent moldability by adding only small loadings equivalent to or less than the conventional ones in the extrusion molding of cement compositions containing the pulp fiber.